In the process of avoiding predation, prey are faced with potentially fitness-compromising trade-offs that have implications for their survival and reproduction. The nature and strength of these non-consumptive effects at the population level can be equivalent, or even greater, than consumptive effects.
Many prey species have evolved defence mechanisms that are induced by predation risk. These inducible defences can be morphological or behavioural in nature. Extensive research has detected these defences in predator–prey communities across freshwater, marine and terrestrial ecosystems. Among this vast research however, an influential portion of these systems has not been widely considered.
Humans inhabit a level in trophic systems above apex predators. In that position, humans have been referred to as a hyperkeystone or super predator species as they have shown a capacity to consume animals at rates many times higher than any other non-human species. However, the extent to which humans induce adaptive defences in animals is not as clear. Systems involving large mammals may be particularly well-suited for the study of human-induced defences given that these species have been disproportionately exploited (for food and competition) over evolutionary time by humans.
To begin this process we first had to examine the context in which large mammals could adaptively evolve inducible defences in relation to human lethality. With the plausibility of these conditions satisfied, we then conducted an extensive review to document the inducible defences that have been detected in large mammals. All of the 187 studies reviewed documented the behavioural plasticity of large mammals to human lethality. No morphological adaptive defences were detected.
However, the extent to which the observed behavioural plasticity of large mammals is representative of adaptive inducible defences remains unclear because the fitness trade-offs (i.e. costs), an integral condition for inducible defences to evolve, were implied rather than quantified among close to 92% of this research. We make recommendations for renewed ingenuity in the development of field experiments that can quantify these costs and discuss the implications of human lethality on the ecology, conservation and management of large mammals.